Toothed power transmission belts such as described in U.S. Pat. No. 2,507,852 issued to R.Y. Case and U.S. Pat. No. 3,756,091 issued to Henry F. Miller have been used for many years for the synchronous transmission of power. To obtain maximum belt life in a synchronous type belt, the teeth must be sufficiently rigid to carry the load and yet able to deflect so as to mate properly with the groove of the pulley. Since the internal strength and rigidity of the tooth elastomer is not sufficient to carry the load by itself and certainly does not itself exhibit sufficient abrasion resistance, a fabric cover is conventionally bonded on the surface of the belt. Historically, the fabric cover which has been employed was made from a so-called "stretchable" nylon fabric. The use of stretchable nylon has made the production of such belts economical according to the method described in my U.S. Pat. No. 3,078,206. Belt constructions, in which fabric is stretched to the shape of the belt teeth have been satisfactory for toothed belts used for synchronous drive purposes. However, as toothed belts are used in systems having greater and greater horsepower requirements, the expected life of belts constructed with stretch fabric has decreased substantially. The reason for this is believed to be the following. The conventional stretchable nylon cover which is made with loosely twisted crimped yarn has its interstices substantially enlarged and filled with tooth rubber during the belt molding process. Vulcanization locks the then stretched fabric in place, causing it to become stiff and consequently reducing its ability to deform under load. Under operating loads the belt teeth are however forced to deflect and subject the thusly stiffened fabric cover to excessive dynamic stresses causing premature rupture of the rubber-filled fabric cover, particularly in the tooth root area, resulting in early belt failure. I believe that in belts made according to may aforesaid Patent, approximately 80% of the load on the belt tooth is transmitted by the fabric and only approximately 20% by the rubber tooth itself. This ratio, I believe, concentrates excessive load on the fabric causing the early belt failure at high horsepower application.
In my copending application, Ser. No. 471,923, I have described a belt in which the teeth are covered with a fabric which is substantially non-stretchable in either the longitudinal or the transverse direction thereof, having been treated with a resin which, when it sets, locks the yarns of the fabric to one another and substantially reduces the amount of elastomeric tooth material which can penetrate the interstices between such yarns, whereby the fabric cover, even though it is intially much stiffer than "stretchable" fabric, retains a greater degree of flexibility after the belt fabrication than would conventional stretchable fabric whose interstices were substantially filled with elastomeric tooth material. This allows more of the load to be transferred to the rubber tooth.
By transferring more of the load from the cover fabric to the rubber tooth itself, the load capacity of the belt now became a function, to a greater degree, of the load carrying capacity of the rubber tooth itself. This made it possible to increase the belt life considerably over that of the known belt constructions, for relatively high horsepower applications.
Another construction for curvilinear belt teeth is disclosed by U.S. Pat. No. 4,392,842. According to that invention, the belt teeth are strengthened against shear under high load by constructing the belt teeth with high internal rigidity. This is accomplished by having an added reinforcing layer suspended internally of the belt tooth with a cushion layer of elastomeric material between the suspended reinforcing layer and the jacket, thereby reinforcing the central portions of the belt teeth while allowing their outer surfaces to be flexible.
The preferred protective outer jacket, according to U.S. Pat. No. 4,392,842, was the conventional stretchable cover fabric and this suffered from many of the disadvantages noted above when used to transmit high horsepower load. That is, a substantial portion of the load was still carried by the jacket fabric and subjected the latter to substantial wear at high horsepower loading. The cushion layer of elastomer, of course, helped to reduce such wear but only to a certain extent. The suspended reinforcement layer rigidified the central tooth portion while the stretchable-fabric jacket cooperated with the cushion layer, to achieve substantial improved belt life over that of conventional belts without suspended reinforcement.
It is an object of the present invention to construct a toothed belt having similarly high horsepower capability and even further substantially improved belt life over the belt life obtainable, at such high horsepower, with known belts constructed with either a stretch-fabric jacket and suspended reinforcement on the one hand, or a non-stretch jacket alone, on the other hand.
It is another object of this invention to provide a a belt construction which overcomes drawbacks of prior constructions and in which fabric material cooperates with the elastomeric material of the belt teeth to provide a belt capable of use for relatively high horsepower applications while exhibiting a belt life substantially longer than can be expected with prior belt constructions under similar operating conditions.